Charles Darwin once speculated that the use of stone tools by our ancient ancestors had profoundly influenced the evolution of our hands. Now, 140 years later, we've discovered that, once again, Darwin knew exactly what he was talking about.

Our hands are fundamentally different from those of our primate relatives. The bones and muscles in our hands feature numerous adaptations that heighten our gripping and manipulatory abilities, none of which the other great apes possess. The other primates have hands that are perfectly suited for locomotion, which makes sense - our cousins are all to a great extent quadrupedal, and most use their hands extensively for climbing. We're pretty much the only primate species that pretty much never uses our hands to move around.

But just because we stopped using our hands for locomotion does not, in itself, explain why we now have such radically different hands. Now University of Kent researchers Dr. Stephen Lycett and Alastair Key think they have the answer: ancient cutting tools fueled the mechanisms of natural selection.

The researchers worked with stone flakes much the same as those used by our ancestors in Africa 2.6 million years ago. They analyzed how the various hand sizes of the test subjects affected the efficiency of the tools as they were used to cut through a rope. This biometric variation, they discovered, revealed a clear and significant relationship between the size and structure of the hand and the test subjects' cutting efficiency.

Intriguingly, these biometric variations were the only significant determinants of efficiency, as the authors report in their paper:

We stress that our results do not imply that tool form has no impact on tool efficiency, but rather that – all other things being equal – biometric variation has a statistically significant influence on efficiency variation when using simple cutting tools. These results demonstrate that biomechanical parameters related directly to efficiency of use, may plausibly have been subject to selection in the earliest stone tool-using hominins.

In announcing these findings, Dr. Lycett observes that Charles Darwin's long ago predictions were right on the money:

"140 years ago, writing from his home at Down House in Kent, Darwin proposed that the use of stone tools may have influenced the evolution of human hands. Our research suggests that he was correct. From a very early stage in our evolution, the cultural behaviour of our ancestors was influencing biological evolution in specific ways."